Phototubes



J. L. WEAVER ET AL PHOTOTUBES Filed Nov. 20, 1953 TTORNEY May 28, 1957 if ff 2 United States Patent O PHOTOTUBES llames L. Weaver and Joseph J. Polko'sky, Lancaster, Pa.,

assignors to Radio Corporation of America, a corporation of Delaware i Application November'20, 1953, Serial No. 393,416

Claims. (Cl. 313-102) This invention relates to phototubes and has vspecial reference to improvements in so-called miniature phototubes, for example of the kind exemplified by U.. S.

Patent 2,506,633 to Engstrom et al.

The miniature phototube of this type comprises an anode cylinder havingan open-end presented across an intervening space to a photocathode consisting of a photosensitive iilm formed on a closed end of a cathode cylinder. The anode and cathode cylinders are concentric and are electrically insulated one from another bya cylindrical glass envelope portion adjacent to the opposite or socket end of the tube. In sealing this insulating envelope portion to the electrode cylinders, instances have occurred wherein the Aheat of the sealing operation was transmitted by conduction, and by convection aircurrents within the anode cylinder, to the photocathode film and created a dead spot (or non-emissive area)v v thereon.

Accordingly, the principal object of the present invention is to provide an improved phototube of thegeneral character described and Wone characterized by its 'substantial'freedom from de-ad' spots in its photocathode.

Another and important object of the invention is' to provide a phototube having an lanode of a construction calculated to afford more efficient collection of photoelectrons than has been achieved in prior devices of the same overall dimensions.

Stated generally, the foregoing and other objects are achieved in accordance with the present invention by substituting la closed-end open-sided, thin-walled anode structure for the open-ended, closed-side, thick-walled anode cylinder of the Engstrom et al. device. The closed end of the improved anode structure serves as a heat shield for the photocathode during the sealing operation incident to the assembly of the tube envelope, thus preventing convection air-currents and the creation of dead spots thereon and, during operation, serves as la most efficient collection surface for the photoelectrons.

The invention is described in greater detail in connection with the accompanying single sheet of drawings wherein:

Fig. 1 is a vertical section of a miniature phototube embodying the invention; and

Fig. 2 is an enlarged view in perspective of the anode of the phototube shown in Fig. l.

The envelope of the miniature phototube shown in Fig. l comprises :a cylindrical cathode portion 1, a cupshape support 3 for an anode structure (later described) and an intermediate insulating cylinder 5, to the opposite ends of which the cathode cylinder 1 and cup-shape support 3 are hermetically sealed, as indicated at 7 and 9, respectively. The free-end of the cathode cylinder 1 contains a window in the form of a glass disk 11 which is coated on the inside with a layer of photosensitive material forming a photocathode 13. In a successfully operated tube of the type described, the photosensitive layer is caesiated antimony. The photosensitive layer or film covers the inner surface of the window 11 and is -patent to Engstrom et al.

Y 2,794,140- Patented May 28, 1957 ICC in permanent contact with the inner surface of the conductive cathode cylinder 1.

The anode, per se, comprises the flattened cross arm or base 15 of an inverted U-shape element 17 (see Fig. 2). The oppositely located parallelly arranged thinmetal arms 19 and 21 of the U are preferably bent or cambered in the direction of the vlong axis of the U so that the free-end of each arm has a snug t against the cylindrical surface of an exhaust tubulation 23 which extends to the exterior of the envelope through the cupshape support. The cambered ends of the arms of the U may be fitted either on the outside or the inside of the tubulation 23 and are preferably bonded thereto as with solder, or by a brazing or welding operation. A hermetic seal about the tubulation 23 at the region where it passes through the cup-shape support 3, may be made in the same way.

Referring again to Fig. 2, the U-shape anode structure 17 is preferably formed from a single strip or sheet of metal (e. g. nickel) about 0.005" thick, with the sheet narrowed adjacent to the region of the cross arm of the U, as indicated at 25, to facilitate the formation of the -at surface of the cross arm 15 and the curved surfaces of its long arms 19 and 21.

A preferred procedure forv assembling the phototube shown in Fig. l is to position the glass window 11 at one end of the cylindrical cathode portion 1 and the insulating cylinder 5 atthe opposite end thereof. While thus positioned the window 11 and insulating cylinder. 5 are then bonded ,to the cylindrical cathode portion, as. shown Yat 27 and 7, by means of high frequency induction seals.

A film of antimony metal forming a part` of the photolcathode 13 is then deposited on the inside surface of the glass window 11, for example by evaporation of the antimony in vacuum and as described in the above cited The tanode structure is then assembled `in a jig (not shown) with the tubulation 23 extending through the cupshape support 3 and the anode arms 19 and 21 fitting around the tubulation 23 and seating on the bottom of the cup-shape support 3. A ring of solder is placed in the bottom of the cup-shape support 3, and the assembly is brazed in an atmosphere of hydrogen.

The cathode assembly above described is then placed over the anode structure with the end of the insulating cylinder 5 resting on the lip of the cup-shape support 3. At this time the tubulation 23 is open at both ends. The envelope is sealed at 9 by means of high frequency induction heating of the cup-shape support 3.

During the making of this seal, by virtue of the novel anode construction no convection air-currents can be set up within the anode and no jet of hot air can be formed and directed against the photocathode 13 lm. In the prior construction, the anode consisted of an extension of the tubulation. Consequently, the heat which Was developed in the cup-shape support to make the seal corresponding to 9 in Fig. l caused air within the closed-sided anode to be heated Iand directed out of the open end of the elongated tubular anode and against the photocathode film, causing destruction of the film, or a dead spot, to occur. Convection currents cannot be set up in the present tube because of the open-sided, closed-ended construction of the anode.

A second source of destructive heating is eliminated by making the anode wall very thin, thereby making the anode a poor heat conductor. The anode wall, made of .005l nickel, is a less efficient heat conductor than the .032 wall copper tubing of the prior construction. Less heat can be conducted from the cup-shape support, which is raised to red heat during sealing, to the end ofthe anode closely adjacent to the photocathode film (the anode-tocathode spacing is about JAGW). Therefore, less heat ranon is gained in the finished mbe.

' the exhaust tubulation 23 by means of a pinch seal 29.

A metal cap or terminal pin 31 is fitted over the tubulation to protect the pinch seal, and thereafter serves to connect the anode of the finished tube into an electrical circuit (not shown).

Another advantage of the closed-ended anode construc- Because the closed end of the present anode can be located closer to the photocathode than the prior open-ended anode without causing a dead spot, the electron collection efficiency is improved, especially at low anode operating voltages.

What is claimed is:

l. A photoelectric tube comprising a cylindrical envelope, a transparent disk closing one end of said envelope, a photosensitive film on the inner surface of said disk for emitting electrons in response to light excitation, and an anode assembly closing the other end of said envelope,

` said Ianode assembly including a thin-metal U-shape element extending within said envelope with its base closely adjacent to said lm for collecting said electrons.V

2. A photoelectric tube comprising a cylindrical envelope, a transparent disk closing one end of said envelope, a photosensitive film on the inner surface of said disk for l emitting electrons in response to light excitation, and an anode assembly sealed to the other end of said envelope, said anode assembly including a relatively thick-walled metal tubulation communicating with the interior of said envelope and a thin-metal open-sided elongated anode joined to said tubulation and extending within said envelope and terminating in a closed end` portion closely adjacent to said lm.

3. A photoelectric tube comprising a cylindrical enve- 1ope,a transparent disk closing one end of said envelope, a photosensitive lm on the inner surface of said disk for emitting electrons in response to light excitation and an anode assembly sealed to the other end of said envelope, said anode assembly including a relatively thick-walled metal tubulation communicating with the interior of said envelope, and a thin-metal U-shape element having its base end closely adjacent to said lm and having its arms supported by said tubulation.

4. A photoelectric tube comprising an envelope including a cylindrical cathode portion, a cup-shape support and an intermediate insulating cylinder, to the opposite ends of which said cathode cylinder and cup-shape support are hermetically sealed, a glass disk sealed to the free end of said cathode cylinder, a photosensitive material on the inner surface of said disk and contacting the inner surface of said cathode cylinder, a thick-walled metal exhaust tubulation communicating with the interior of said envelope and extending through said cup-shape support and sealed thereto, and a thin-metal U-shape anode within said envelope, said anode h-aving the ends of its arms fitted to said exhaust tubulation and bonded thereto, said arms extending within said envelope and terminating in a at cross-arm adjacent to said photosensitive lm.

5. An electron discharge device comprising a tubular envelope, a transparent Window closing one end of said envelope, a layer of a material on the inner surface of said Window capable of emitting electrons in response to radiant energy excitation, and an anode assembly closing the other end of said envelope, said anode assembly including an elongated element of open walled construction extending within said envelope and terminating in a closed end electron collecting surface portion closely adjacent to said electron emitting layer.

References Cited in the tile of this patent UNITED STATES PATENTS 2,092,206 Dudding Sept. 7, 1937 2,506,633 Engstrorn et al. May 9, 1950 2,613,330 Bruining et al. Oct. 7, 1952 

